This invention relates to silver halide photography. More specifically, the invention relates to an improved method for preparing tabular grain silver halide emulsions with uniform epitaxial depositions, and to photographic elements which contain one or more of such improved emulsions.
Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having a diameter of at least 0.6 xcexcm and a thickness of less than 0.3 xcexcm exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional sensitizations, such as reduction sensitization were also disclosed.
Maskasky U.S. Pat. No. 4,435,501 discloses that use of a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, adsorbed to the surfaces of host tabular grains is capable of directing silver salt epitaxy to selected sites, typically the edges and/or corners, of the host grains. Depending upon the composition and site of the silver salt epitaxy, significant increases in speed may be observed. The most highly controlled site depositions (e.g., corner specific epitaxy siting) and the highest reported photographic speeds reported by U.S. Pat. No. 4,435,501 were obtained by epitaxially depositing silver chloride onto silver iodobromide tabular grains. U.S. Pat. No. 4,435,501 recognized that even when chloride is the sole halide run into a tabular grain emulsion during epitaxial deposition, a minor portion of the halide contained in the host tabular grains can migrate to the silver chloride epitaxy. U.S. Pat. No. 4,435,501 offers as an example the inclusion of minor amounts of bromide ion when silver and chloride ions are being run into a tabular grain emulsion during epitaxial deposition.
In Daubendiek et al. U.S. Pat. No. 5,576,168, sensitized silver iodobromide ultrathin emulsions are disclosed, wherein during sensitization silver and halide ions including iodide and chloride ions are added to ultrathin tabular host grains to deposit epitaxially on up to 50 percent of the surface area of the tabular grains silver halide protrusions containing at least a 10 mole percent higher chloride concentration than the tabular grains and an iodide concentration that is increased by the iodide ion addition. The resulting epitaxially sensitized ultrathin tabular grain emulsions are observed to provide increased speed and contrast as well as improvements in speed-granularity relationships.
It is generally accepted that selective site deposition of silver halide epitaxy onto host tabular grains improves sensitivity by reducing sensitization site competition for conduction band electrons released by photon absorption on imagewise exposure. Thus, epitaxy over a limited portion of the major faces of the ultrathin tabular grains is more efficient than that overlying all or most of the major faces, still better is epitaxy that is substantially confined to the edges of the host tabular grains, with limited coverage of their major faces, and still more efficient is epitaxy that is confined at or near the corners or other discrete sites of the tabular grains. The spacing of the corners of the major faces of the host tabular grains may in itself reduce photoelectron competition sufficiently to allow near maximum sensitivities to be realized. U.S. Pat. No. 4,435,501 teaches that slowing the rate of epitaxial deposition can reduce the number of epitaxial deposition sites on a host tabular grain. Yamashita et al U.S. Pat. No. 5,011,767, here incorporated by reference, carries this further and suggests specific spectral sensitizing dyes and conditions for producing a single epitaxial junction per host grain. It has been observed, however, that attemps to limit the number of epitaxial deposit sites often leads to grain to grain inconsistencies, which can lead to less than desired photographic performance.
While various techniques for directing epitaxial depositions on tabular host grains have been previously described, it would be desirable to provide further improved processes for uniformly siting epitaxial depositions on the corners of host tabular grains, particularly in the case where the epitaxial depositions comprise a relatively low molar percent based on the total silver of the host grains (e.g., from 0.5 to 7 mole percent). At such low epitaxial deposition range, it has been observed that most grains do not have epitaxial depositions on the majority of their grain corners, and the number of corners on which epitaxial deposition does occur frequently varies from grain to grain.
It is also known in the art that when fog is generated in the precipitation stage, certain compounds which act as oxidizing agents for silver can be added during the grain-forming process to reduce the undesirable minute silver clusters that constitute this fog. These agents include hydrogen peroxide, peroxy acid salts, disulfides (U.S. Pat. No. 3,397,986), mercury compounds (U.S. Pat. No. 2,728,664), iodine (EP 576,920), iodide releasing agents (EP 563,708, EP 562,476, EP 561,415, and JP06,011,784) and p-quinone (U.S. Pat. No. 3,957,490). The use of thiosulfonate compounds for controlling fog during precipitation has been claimed in the following U.S. patents: U.S. Pat. No. 5,061,614, U.S. Pat. No. 5,079,138; U.S. Pat. No. 5,244,781; U.S. Pat. No. 5,185,241; and U.S. Pat. No. 5,229,263. Likewise, in the following European applications, EP 368,304; EP 435,355; and EP 435,270, the use of thiosulfonates during grain formation of AgX emulsions is claimed.
U.S. Pat. No. 5,244,781 discloses the preparation of silver halide emulsions having at least one structure resulting from differences in halogen compositions, where such emulsions are prepared in the presence of an oxidizing agent for silver. Described grains structures having differences in halogen compositions include layered structures (core-shell) as well as grains having epitaxial depositions. The oxidizing agents are described as compounds which can act on metal silver and convert it into silver ions or react with reducing impurities capable of producing fog centers. Various oxidizing agents are proposed, including thiosulfonate compounds, but there is no teaching as to the impact of any of such oxidizing agents on the directing of uniform epitaxial depositions on host tabular grains.
It would be desirable to provide a process for epitaxially sensitization of tabular grain emulsions with relatively low molar levels of epitaxy depositions, wherein the depositions are more uniformly deposited on the majority of corners of the host tabular grains.
In one aspect this invention is directed towards a process of sensitizing a tabular grain emulsion containing the steps of
(i) providing a tabular grain host emulsion comprised of a dispersing medium and silver halide grains including tabular grains having {111} major faces and an aspect ratio of at least 2, which contain greater than 50 mole percent bromide, based on silver, and which account for greater than 50 percent of total grain projected area;
(ii) adding a thiosulfonate compound of the following Formula I at the surface of the tabular grains of the host emulsion:
Z1SO2SZ2xe2x80x83xe2x80x83(I)
where Z1 represents is a substituted or unsubsituted aliphatic, aromatic, or heterocyclic group, and Z2 represents a substituted or unsubsituted aliphatic, aromatic, or heterocyclic group, or a monovalent metal or organic cation, where Z1 and Z2 may combine together to form a ring structure or either of Z1 or Z2 may comprise a polymeric backbone wherein the thiosulfonate group may be repeated, and
(iii) adding silver and halide ions, where the halide ions include at least chloride ions, to the tabular grain host emulsion in an amount of from 0.5-7 mole percent based on total silver of the tabular grain host emulsion to precipitate silver halide protrusions forming epitaxial junctions with up to 50 percent of the surface area of the tabular grains essentially at only the corners of the tabular grains, the protrusions having an isomorphic face centered cubic crystal structure and including at least a 10 mole percent higher chloride ion concentration than the host tabular grains.
In another aspect, this invention is directed towards an improved radiation-sensitive emulsion comprised of
(i) a dispersing medium
(ii) silver halide grains including host tabular grains having {111} major faces and an aspect ratio of at least 2, which contain greater than 50 mole percent bromide, based on silver, and which account for greater than 50 percent of total grain projected area,
(iii) a thiosulfonate compound of the following formula (I):
Z1SO2SZ2xe2x80x83xe2x80x83(I)
where Z1 represents is a substituted or unsubsituted aliphatic, aromatic, or heterocyclic group, and Z2 represents a substituted or unsubsituted aliphatic, aromatic, or heterocyclic group, or a monovalent metal or organic cation, where Z1 and Z2 may combine together to form a ring structure or either of Z1 or Z2 may comprise a polymeric backbone wherein the thiosulfonate group may be repeated;
(iv) a spectral sensitizing dye adsorbed to the surfaces of the tabular grains, and
(v) latent image forming chemical sensitization sites on the surfaces of the tabular grains comprising epitaxially deposited silver halide protrusions exhibiting an isomorphic face centered cubic crystal lattice structure and containing a silver chloride concentration at least 10 mole percent higher than that of the tabular grains, wherein the epitaxial protrusions comprise from 0.5-7 mole percent based on total silver of the host tabular grains and form epitaxial junctions with up to 50 percent of the surface area of the tabular grains and at the majority of corners of the tabular grains.
In a further aspect, this invention is directed towards a photographic element comprised of a support, and a silver halide emulsion layer coated on the support comprised of an emulsion as described herein.
In preferred embodiments of the invention, the epitaxially deposited silver halide protrusions of the tabular grain emulsion comprise from 1-6 mole percent, more preferably 3-6 mole percent, based on total silver of the host tabular grains.
The results of the invention employing specific epitaxial sensitization deposits are an improvement over the position demonstrated by the use of epitaxially sensitized tabular grain emulsions outside the invention.